1. Field of the Invention
The present invention relates generally to an emergency core cooling system (ECCS) for a pressurized light water reactor, which directly injects emergency core cooling water, which is supplied from a high-pressure safety injection pump or a safety injection tank, into the downcomer of a reactor vessel, and, more particularly, to a downcomer injection extension duct technology for interrupting an emergency core cooling water direct bypass discharge phenomenon in which emergency core cooling water is guided by a strong cross-flow of the downcomer in the event of a large break loss of coolant accident (LBLOCA), and is then discharged out of the reactor.
2. Description of the Related Art
A pressurized light water reactor can encounter unexpected safety problems even though it has been designed in consideration of a sufficient safety margin. If sufficient emergency core cooling water is not supplied when a safety problem in which a large quantity of cooling water leaks occurs, a core can overheat, resulting in damage to the reactor. In order to cool the core when the cooling water leaks, the pressurized light water reactor is equipped with a high-pressure safety injection pump and a safety injection tank such that the emergency core cooling water is exhausted externally. The supply of the emergency core cooling water is divided into two types according to the position of the injection nozzle end. Among the two types, one is a cold leg injection type, in which the injection nozzle is located at a cold leg, and the other is a direct vessel injection type, in which the injection nozzle is located at a reactor vessel.
The cold leg injection type means that the emergency core cooling water is supplied to a reactor system through an injection line, which is connected to a cold leg corresponding to a pipe supplying cold water from the circulating pump of a reactor coolant circulatory system into the reactor vessel, and has a drawback in that, when the emergency core cooling water is supplied to a broken cold leg, the emergency core cooling water completely leaks out of the broken cold leg, and thus the reactor core cooling effect cannot be expected. As such, the direct vessel injection type is currently configured to include a direct vessel injection (DVI) nozzle, which supplies the emergency core cooling water to the reactor vessel, and to directly supply the emergency core cooling water to a downcomer between the reactor vessel and a core support barrel.
However, the direct vessel injection type has a problem in that there is an increase in the emergency core cooling water direct bypass phenomenon, in which, when the cold leg is broken, the emergency core cooling water is headed to the broken cold leg by strong cross-flow of the downcomer, and is discharged out of the reactor vessel. As illustrated in FIGS. 1A through 1D, a conventional technology for preventing the emergency core cooling water direct bypass phenomenon is designed such that an injection extension duct 110 or 110′ is installed on the outer surface of a core barrel 100 of the downcomer 140 of FIG. 1A or on a baffle region in a core barrel 100 of FIG. 1B, and such that the injection extension duct 110 or 110′ is connected with the DVI nozzle 120 across the downcomer 140 using a pipe 130. Further, the conventional technology is designed such that an injection extension duct 210 is installed on the outer surface of a core barrel 200 of the downcomer 240 of FIG. 1C, and such that the injection extension duct 210 is connected with the DVI nozzle 220 across the downcomer 240 using projection nozzles 230 and 230′.
As disclosed in U.S. Pat. Nos. 5,377,242 (James D. Carlton, et al.) and 5,135,708 (James D. Carlton, et al.), illustrated in FIGS. 1A and 1B respectively, the conventional method of connecting the DVI nozzle 120 with the injection extension duct 110 or 110′ in the downcomer 140 using the pipe 130 entails difficulty in the installation of the connecting portion because the gap in the downcomer 140 is narrow. When the reactor vessel is assembled with the core barrel, interference between the projections occurs. Further, according to this prior art, when a large cold leg 150 is broken, the emergency core cooling water can be effectively injected up to a lower portion or a core inlet of the downcomer 140. However, when a DVI line itself is broken, an outlet of the injection extension duct 110 or 110′ functions as an inlet of a break flow due to a siphon effect, and the level of the cooling water in the reactor vessel is lowered by the length of the injection extension duct 110 or 110′, so that the core cladding temperature is abnormally increased. This leads to a problem of noncompliance with safety regulations.
As illustrated in FIG. 1C, another conventional technology similar to the aforementioned technology is adapted to directly connect the DVI nozzle 220 and the injection extension duct 210 in the downcomer 240 using a pipe, to position the protruding nozzles 230 and 230′ so as to be opposite to each other, and form a slight gap (Korean Patent Application Publication No. 10-2000-0074521). However, this conventional technology also has a problem in that, when the reactor vessel is assembled with the core barrel 200, interference between the nozzles 230 and 230′ protruding to the downcomer 240 occurs, thus making an assembly difficult, and thus a hole, which is used for a periodical withdrawal checkup of a neutron monitoring capsule installed at a lower portion of the reactor vessel, overlaps with the protruding nozzles, so that work becomes impossible. Further, when the DVI pipe line is broken, the gap between the upper connection nozzles of the injection extension duct 210 is narrow, and thus an inlet-outlet reverse phenomenon, in which the lowest outlet of the injection extension duct 210, located at the lowest position of the injection extension duct 210, functions as an inlet, is caused, although the quantity of intake of the break flow is not much. As such, there is a problem in that the level of the cooling water in the reactor vessel is significantly lowered down to the lowest outlet of the injection extension duct 210, and then the lowest outlet of the injection extension duct 210 functions as an inlet.
There is a simpler technology in which an outlet of the DVI nozzle is vertically positioned at a right angle using an elbow 320 (Korean Patent Application Publication No. 10-2003-0064634). However, since the space occupied by the elbow 320 is similar to the gap in the downcomer 330, the reactor vessel 300 cannot be assembled with the core barrel 310. As a result of performing an emergency core cooling water bypass test, it was found that this simple vertical injection has little thermal hydraulic effect, because the direct bypass rate of the emergency core cooling water is very high (NED Vol. 225, “Effect of the yaw injection angle on the ECC bypass in comparison with the horizontal injection,” T. S. Kwon et al., 2003).
According to the aforementioned conventional technology, the DVI line for the emergency core cooling water is broken, and thus the lowest outlet of the injection extension duct functions as an inlet for the break flow. In this case, the level of the cooling water in the reactor vessel is gradually lowered to reach a position equal to or lower than the lowest outlet of the injection extension duct, which is located at the lowest position of the injection extension duct. When the level of the cooling water is lowered, the reactor core is exposed. This has a lethal result when the reactor core is cooled.
As described above, the conventional common technical problem is mostly attributable to a connection structure in which the DVI nozzle and the injection extension duct are connected to each other in the downcomer. Thus, in order to improve the assemblability between the reactor vessel and the core barrel, avoid an interference between the structures within a checkup work area during the operation, interrupt the inlet-outlet reverse phenomenon of an injection extension duct when the DVI line is broken, and avoid interference between the withdrawal inlet of the neutron monitoring capsule and the injection extension duct or the protruding nozzles, the concept of an injection extension duct having a new structure is required.
An emergency core cooling water direct vessel injection system, which directly injects emergency core cooling water into the downcomer of a reactor vessel in a pressurized light water reactor complies with the following design requirements.
First, the emergency core cooling water direct vessel injection system should be able to supply a larger quantity of emergency core cooling water to a core inlet through a lower portion of the downcomer by interrupting a phenomenon in which the emergency core cooling water is bypassed and discharged by a high-speed steam cross flow in the downcomer occurring in the event of a large break loss of coolant accident (LBLOCA).
Second, a phenomenon in which the level of the cooling water in the reactor vessel is significantly reduced should not occur because an emergency core cooling water outlet of the lowest position of the injection extension duct functions as an inlet for a break flow so as to be able to be applied when the pipe of a direct vessel injection system is broken.
Third, due to the injection extension duct installed in the downcomer, the cross flow resistance should not be excessively increased, and the flow induced vibration should not be excessively increased.
Fourth, a connector of the injection extension duct and the direct vessel injection nozzle should not cause an interference when the reactor vessel is assembled with the core barrel in the downcomer or an interference with the withdrawal hole of a neutron monitoring capsule. Thereby, a practical application is possible, and the design can be certified, and continuous checkup during an operation of the reactor is possible.